Black smoke billows from burning tires at a checkpoint following an attack by Ukrainian troops outside Slovyansk, Ukraine, Thursday, April 24, 2014. Ukrainian government troops moved against pro-Russia forces in the east of the country on Thursday and killed at least two of them in clashes at checkpoints manned by the insurgents, the government and insurgents said. Russian President Vladimir Putin decried what he described as a "punitive operation." (AP Photo/Mika Velikovskiy)

SLOVYANSK, Ukraine — Ukrainian troops moved against pro-Russia forces in eastern Ukraine on Thursday, killing at least two of them in clashes at checkpoints, both sides said. Russian President Vladimir Putin decried what he described as a “punitive operation.”

The fighting was the first since acting President Oleksandr Turchynov on Tuesday ordered the resumption of military operations in eastern Ukraine, where pro-Russia protesters and masked gunmen have seized government buildings in at least 10 cities and set up roadblocks.

In St. Petersburg, Putin threatened Kiev with unspecified consequences.

“If the Kiev government is using the army against its own people this is clearly a grave crime,” Putin said.

The Ukrainian government and the West, who have accused Russia of directing and supporting the insurgents, worry that Putin would welcome a pretext for a military intervention in eastern Ukraine. Putin denies that any Russian agents are operating in Ukraine, but insists he would have the right to intervene to protect the ethnic Russians who make up a sizeable minority in the east.

Russia has tens of thousands of troops arrayed along its side of Ukraine’s eastern border.

Earlier in Tokyo, President Barack Obama accused Moscow of failing to live up to “the spirit or the letter” of a deal last week to ease tensions in eastern Ukraine. If that continues, Obama said, “there will be further consequences and we will ramp up further sanctions.”

The Ukrainian Interior Ministry said military and special police forces killed “up to five terrorists” while destroying three checkpoints north of Slovyansk, a city 160 kilometers (100 miles) west of the Russian border that has emerged as the focus of the armed insurgency. One government force member was injured, the statement said.

Stella Khorosheva, a spokeswoman for the Slovyansk insurgents, said two pro-Russia fighters were killed at a checkpoint in the village of Khrestyshche, 10 kilometers (six miles) north of the city. She said checks were being made at hospitals to see if there were other casualties.

She later claimed that the pro-Russian militia had regained control over the checkpoints that had been attacked by the Ukrainian troops.

The situation was quiet in Slovyansk itself, but checkpoints inside the city were abandoned and it was unclear where the pro-Russia insurgents manning them had gone.

Khorosheva said the fighters were ready to repel any attack by government troops.

“We will defend ourselves to our last drop of blood. We are ready to repeat Stalingrad,” she told The Associated Press, invoking the memory of the Soviet army’s victory over German forces in 1942-43.

At least 10 Ukrainian government armored vehicles were seen parked on the road north of Slovyansk and two helicopters were circling over the area. Troops ordered residents in the area to keep away.

Near the town of Makatikha, several miles (kilometers) north of Slovyansk, pro-Russia militia set fire to barricades of car tires in an apparent attempt to reduce the visibility from the air. An Associated Press reporter saw about two dozen militiamen manning checkpoints along the road earlier in the day.

In the southeastern city of Mariupol, Interior Minister Arsen Avakov said his forces had cleared city hall of the pro-Russia protesters who had been occupying it for more than a week. The Ukrainian city sits along the main road between Russia and Russia’s newly annexed Black Sea peninsula of Crimea.

Avakov wrote in a Facebook post that Mariupol city hall “has been freed to resume work” but did not describe the operation.

Yulia Lasazan, a spokeswoman for Mariupol’s police department, told The Associated Press about 30 masked men armed with baseball bats stormed the building in the dark early Thursday and started beating the protesters. Five people were taken to a hospital, she said.

It was not clear why the protesters, some of whom were believed to be armed, did not offer resistance but called local police instead. Lasazan said the police were controlling Mariupol city hall’s perimeter and were negotiating with the remaining protesters to leave the building.

Ukraine is going through its biggest political crisis since the fall of Soviet Union, set off by months of anti-government protests that led to President Viktor Yanukovych’s flight to Russia in February.

Yanukovych’s ouster sparked wide anger in his support base in Ukraine’s east. The insurgents, who claim Ukraine’s post-Yanukovych government consists of nationalists who will suppress the east, are demanding regional autonomy or even annexation by Russia.

Ukraine and Russia reached a deal in Geneva last week to defuse the crisis, but pro-Russian insurgents in the east — and right-wing militants in Kiev — have defied calls for all sides to disarm and to vacate the buildings they are occupying.

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